Laser annealing method

ABSTRACT

In crystallizing an amorphous silicon film by illuminating it with linear pulse laser beams having a normal-distribution type beam profile or a similar beam profile, the linear pulse laser beams are applied in an overlapped manner. There can be obtained effects similar to those as obtained by a method in which the laser illumination power is gradually increased and then decreased in a step-like manner in plural scans.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 10/602,762, filed Jun. 25, 2003, now U.S. Pat. No. 6,947,452 nowallowed, which is a continuation of U.S. application Ser. No.08/594,670, filed Feb. 2, 1996, now U.S. Pat. No. 6,596,613, and claimsthe benefit of foreign priority applications filed in Japan as SerialNo. 07-037705 on Feb. 2, 1995 and as Serial No. 07-068670 on Mar. 2,1995. This application claims priority to each of these priorapplications, and the disclosures of the prior applications areconsidered part of (and are incorporated by reference in) the disclosureof this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique of annealing, for instance,a semiconductor material uniformly and efficiently over a large area.The invention also relates to a technique of preventing reduction ofprocessing efficiency in illuminating a particular region whilegradually changing the illumination energy density.

2. Description of the Related Art

In recent years, extensive studies have been made of the temperaturereduction of semiconductor device manufacturing processes. This islargely due to the need of forming semiconductor devices on aninsulative substrate, such as a glass substrate, which is inexpensiveand superior in workability. Other needs such as needs of forming finerdevices and multilayered devices have also prompted the studies on theprocess temperature reduction.

In particular, a technique of forming semiconductor devices on a glasssubstrate is necessary to produce a panel that constitutes an activematrix liquid crystal display device. This is a configuration in whichthin-film transistors are formed on a glass substrate so as to assume amatrix of more than several hundred by several hundred. When a glass isexposed to an atmosphere of more than about 600° C., deformation such ascontraction and strain becomes remarkable. Therefore, the heatingtemperature in a thin-film transistor manufacturing process should be aslow as possible.

To obtain thin-film transistors having superior electricalcharacteristics, a crystalline thin-film semiconductor needs to be used.

Among methods of producing a crystalline silicon film is a technique ofcrystallizing, by a heat treatment, an amorphous silicon film that hasbeen deposited by plasma CVD or low-pressure thermal CVD of about 500°C. This heat treatment is such that a sample is left in an atmosphere of600° C. or more for more than several hours. In this heat treatment,where the temperature is, for instance, 600° C., long process time ofmore than 10 hours is needed. In general, if a glass substrate is heatedat 600° C. for more than 10 hours, deformation (strain and contraction)of the substrate becomes remarkable. Since a thin-film semiconductor forconstituting thin-film transistors is several hundred angstrom inthickness and several micrometers to several tens of micrometers insize, the substrate deformation will cause an operation failure, avariation in electrical characteristics, or the like. In particular, inthe case of a large-sized substrate (diagonal size: 20 inches or more),the substrate deformation is a serious problem.

If the heat treatment temperature is higher than 1,000° C.,crystallization can be attained in a process time of several hours.However, ordinary glass substrates cannot withstand a high temperatureof about 1,000° C. even if a heat treatment lasts for a short time.

Quartz substrates can withstand a heat treatment of more than 1,000° C.,and allow production of a silicon film having superior crystallinity.However, large-area quartz substrates are particularly expensive.Therefore, from the economical point of view, they cannot be easilyapplied to liquid crystal display devices, which will be required to beincreased in size in the future.

In the above circumstances, the temperature of processes formanufacturing thin-film transistors is now required to be lowered. Amongtechniques for attaining this purpose is an annealing technique thatuses laser light illumination, which technique now attracts muchattention with a possibility of providing an ultimate low-temperatureprocess. Since laser light can impart as high energy as thermalannealing to only a necessary portion, it is not necessary to expose theentire substrate to a high-temperature atmosphere. Therefore, theannealing technique by laser light illumination enables use of glasssubstrates.

However, the annealing technique by laser light illumination has aproblem of unstable laser light illumination energy. Although thisproblem can be solved by using a laser apparatus capable of emittinglaser light of higher energy than necessary one and attenuating theoutput laser light, there remains another problem of cost increase dueto increased size of the laser apparatus.

Even with such a problem, the annealing technique by laser lightillumination is still very advantageous in that it enables use of glasssubstrates.

In general, there are two laser light illumination methods describedbelow.

In a first method, a CW laser such as an argon ion laser is used and aspot-like beam is applied to a semiconductor material. A semiconductormaterial is crystallized such that it is melted and then solidifiedgradually due to a sloped energy profile of a beam and its movement.

In a second method, a pulsed oscillation laser such as an excimer laseris used. A semiconductor material is crystallized such that it isinstantaneously melted by application of a high-energy laser pulse andthen solidified.

The first method of using a CW laser has a problem of long processingtime, because the maximum energy of the CW laser is insufficient andtherefore the beam spot size is at most several millimeters by severalmillimeters. In contrast, the second method using a pulsed oscillationlaser can provide high mass-productivity, because the maximum energy ofthe laser is very high and therefore the beam spot size can be madeseveral square centimeters or larger.

However, in the second method, to process a single, large-area substratewith an ordinary square or rectangular beam, the beam needs to be movedin the four orthogonal directions, which inconvenience still remains tobe solved from the viewpoint of mass-productivity.

This aspect can be greatly improved by deforming a laser beam into alinear shape that is longer than the width of a subject substrate, andscanning the substrate with such a deformed beam.

The remaining problem is insufficient uniformity of laser lightillumination effects. The following measures are taken to improve theuniformity. A first measure is to make the beam profile as close to arectangular one as possible by causing a laser beam to pass through aslit, to thereby reduce an intensity variation within a linear beam. Asecond measure to further improve the uniformity is to performpreliminary illumination with pulse laser light that is weaker than thatof subsequently performed main illumination. This measure is soeffective that the characteristics of resulting semiconductor devicescan be improved very much.

The reason why the above two-step illumination is effective is that asemiconductor material film including many amorphous portions has alaser energy absorption ratio that is much different than apolycrystalline film. For example, a common amorphous silicon film (a-Sifilm) contains hydrogen at 20 to 30 atomic percent. If laser lighthaving high energy is abruptly applied to an amorphous silicon film,hydrogen is ejected therefrom, so that the surface of the film isroughened, i.e., formed with asperities of several tens of angstrom toseveral hundred angstrom. Since a thin-film semiconductor for athin-film transistor is several hundred angstrom in thickness, itssurface having asperities of several tens of angstrom to several hundredangstrom will be a major cause of variations in electricalcharacteristics etc.

Where the two-step illumination is performed, a process proceeds suchthat a certain part of hydrogen is removed from an amorphous siliconfilm by the weak preliminary illumination and crystallization iseffected by the main illumination. Since the illumination energy is nothigh in the preliminary illumination, there does not occur severesurface roughening of the film due to sudden hydrogen ejection.

The uniformity of the laser light illumination effects can be improvedconsiderably. However, if the above two-step illumination is employed,the laser processing time is doubled, thus reducing throughput. Further,since a pulsed laser is used, some variation occurs in the laserannealing effects depending on the registration accuracy of the mainillumination and preliminary illumination, which variation may greatlyinfluence the characteristics of thin-film transistors having a size ofseveral tens of micrometers by several tens of micrometers.

In general, among various processing techniques (for example, causing aquality change in various materials and processing by application oflaser energy) by laser light illumination is a technique in which acertain region is illuminated plural times with laser beams of variedenergies. The above-described annealing technique for a silicon film isan example of such a technique.

Conventionally, in such a technique, a laser beam is applied pluraltimes, which however elongates the processing time by a factor of thenumber of illumination times, and causes a large decrease in theoperation efficiency. Further, illuminating a particular region pluraltimes with laser beams likely causes a problem of a deviation ofillumination areas, and is not practical because solving this problemmay be technically difficult or may require a costly technique.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the problem ofnonuniformity of the effects of annealing by laser light illumination.Another object of the invention is to improve the economy of laser lightillumination.

The invention solves the above problems by devising a new energy profileof a linear laser beam, which profile varies continuously or step-likemanner. Specifically, a normal-distribution type profile or atrapezoidal profile is employed.

To solve the above problems, one aspect of the invention ischaracterized in that an illumination object is illuminated with pulselaser beams that have been shaped into linear beams while being scannedwith the laser beams relatively in one direction.

For example, as shown in FIG. 3, a semiconductor material isilluminated, while being scanned, with a laser beam having anormal-distribution type energy profile in its width direction (i.e.,scanning direction). With this illumination method, a foot-to-middleportion of the normal-distribution type profile corresponds to thepreliminary illumination having low laser beam energy, while amiddle-to-top portion of the profile corresponds to the mainillumination having high energy. Therefore, a single laser beamilluminating operation can provide effects similar to those obtained bythe two-step or multi-step laser beam illumination. Alternatively, asshown in FIG. 5, a semiconductor material is illuminated with a laserbeam having a trapezoidal energy profile in its width direction(scanning direction). In this case, a slope portion of the trapezoidalprofile has a function of imparting energy corresponding to that of thepreliminary illumination, while a top base portion of the profile has afunction of imparting energy corresponds to that of the mainillumination.

Another aspect of the invention is characterized in that an illuminationobject is illuminated with pulse laser beams that have been shaped intolinear beams while the laser beams are moved in one direction, in whichthe laser beams are applied in an overlapped manner so that anarbitrarily selected point on the illumination object is illuminatedplural times.

In this method, a particular region is illuminated with laser beamsplural times by applying linear laser beams in an overlapped manner.

In particular, where laser beams having a normal-distribution typeenergy profile (see FIG. 3) or a trapezoidal energy profile (see FIG. 5)in the scanning direction are applied in an overlapped manner whilebeing moved little by little, in a particular linear region the appliedenergy density first increases continuously or in a step-like manner andthen decreases continuously or in a step-like manner. Therefore, thismethod can provide effects similar to those obtained by the two-step ormulti-step laser light illumination.

To provide effects equivalent to those obtained by the multi-stepillumination, the number of overlapping of laser beam pulses may be setat 3 to 100, preferably 10 to 30.

However, to obtain necessary annealing effects, it is preferred that thelaser beam illumination is so performed as to satisfy certainconditions, which are:

(1) The illumination object is a silicon film of 150 to 1,000 Å inthickness.

(2) The laser beams are pulse beams having a pulse rate of N per second,assumes a linear shape having a width L, and has a beam profile in whichthe energy density varies continuously or in a step-like manner in thewidth direction.

(3) The laser beams are applied to an illumination surface while beingmoved at a speed V in the width direction.

(4) The average single-pulse energy density is set at 100 to 500 mJ/cm².

(5) The laser beams are applied so as to satisfy a relationship10≦LN/V≦30.

A laser beam illumination method of the invention which satisfies theabove conditions is described as comprising the steps of:

emitting pulse laser beams at a rate of N times per second;

shaping the pulse laser beams into linear beams having a width L, anenergy profile that varies continuously or in a step-like manner in awidth direction thereof, and an average single-pulse energy density of100 to 500 mJ/cm²; and

applying the laser beams to a silicon film having a thickness of 150 to1,000 Å while scanning it with the laser beams in the width direction ata speed V so as to satisfy a relationship 10≦LN/V≦30.

Among the above conditions, the condition that the illumination objectis a silicon film of 150 to 1,000 Å in thickness is established for thefollowing reasons. Experiments have shown that in annealing of a siliconfilm, if the thickness of the silicon film is less than 150 Å, theuniformity of film formation, the uniformity of annealing effects, andthe reproducibility are insufficient. On the other hand, a silicon filmhaving a thickness of more than 1,000 Å is not practical because itrequires a large-output laser. In addition, a crystalline silicon filmhaving such a thickness is not used for a thin-film transistor.

Examples of the laser beam having a beam profile that variescontinuously or in a step-like manner in the width direction are a laserbeam having a normal-distribution type energy profile in the scanningdirection (see FIG. 3) and a laser beam having a trapezoidal energyprofile in the scanning direction (see FIG. 5).

The reason for employing the energy density of 100 to 500 mJ/cm² is thatexperiments have revealed that laser annealing of a silicon film havinga thickness of not more than 1,000 Å can be performed effectively byusing a laser beam of the above energy density. The energy density asused above is defined as the value of a top portion of a profile thatvaries continuously or in a step-like manner. For example, in the caseof a normal-distribution type profile, the energy density is defined asthe maximum value. In the case of a trapezoidal profile, the energydensity is defined as the value of a top base portion.

In the above method, the parameter LN/V represents the number of laserbeam pulses applied to a particular linear region when it is scannedwith linear pulse laser beams once. To attain the effects as obtained bythe multi-step illumination, it is preferred that the number of laserbeam pulses be set at 10 to 30.

According to a further aspect of the invention, there is provided alaser beam illumination method comprising the steps of:

emitting pulse laser beams at a rate of N times per second;

shaping the pulse laser beams so that they have an energy profile inwhich an energy density varies continuously or in a step-like mannerover a length L in a predetermined direction; and

applying the laser beams to a predetermined region while scanning itwith the laser beams in the predetermined direction at a speed V,wherein the number n of laser beam pulses applied to the predeterminedregion in one scan satisfies a relationship n=LN/V.

By employing the above method, a particular region can be illuminated ntimes with laser beams whose energy density gradually varies.

In the invention, the laser beam illumination energy profile is notlimited to normal-distribution type and trapezoidal profiles. Forexample, there may be employed a beam shape in which the energy densityvaries in a step-like manner, or a triangular energy profile may beused.

For example, when a linear laser beam having a normal-distribution typeenergy profile as shown in FIG. 3 is applied while being moved forscanning in its width direction so that certain conditions aresatisfied, first a weak foot portion of the energy profile is appliedand the illumination energy gradually increases. After a portion havinga certain energy value is applied, the illumination energy graduallydecreases and the illumination is finished.

For example, when linear pulse laser beams having a normal-distributiontype illumination energy profile in the width direction are used and acondition LN/V=15 is satisfied where L is a beam width, N is the numberof emissions per second, and V is a scanning speed, a linear region isilluminated with 15 laser beam pulses in one laser beam scan.

The 15 laser beam pulses, which are sequentially applied, have energydensity values of 15 sections of the normal-distribution type profile,respectively. For example, a particular linear region (the width of thisregion is very narrow) is sequentially illuminated with laser beampulses having energy density values E₁ to E₁₅ shown in FIG. 4. As thelaser beam pulses of E₁ to E₈ are sequentially applied, the illuminationenergy density gradually increases. On the other hand, as the laser beampulses of E₈ to E₁₅ are sequentially applied, the illumination energydensity gradually decreases.

A process of this type in which first the illumination energy isgradually increased and then gradually decreased can attain desiredannealing effects while suppressing surface roughening of a siliconfilm. Further, since desired effects can be obtained by laser beamillumination of one scan rather than plural times of laser beamilluminating operations, high operation efficiency can be attained.

In particular, effects similar to those as obtained by the multi-stepillumination can be attained by using laser beams whose energy densityis varied continuously. This function similarly applies to effects otherthan the annealing effects on a silicon film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general configuration of an apparatus for laser beamillumination;

FIG. 2 shows an optics for shaping a laser beam into a linear form;

FIG. 3 schematically illustrates scanning with a linear laser beamhaving a normal-distribution type energy profile;

FIG. 4 is a graph showing a general form of a normal-distribution typeenergy profile; and

FIG. 5 schematically illustrates scanning with a linear laser beamhaving a trapezoidal energy profile.

FIG. 6 is an irradiation energy profile in accordance with the presentinvention.

FIG. 7 is an irradiation energy profile in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

In this embodiment, a silicon film is used as a semiconductor material.There is a tendency that in the process of enhancing the crystallinityof an amorphous or crystalline silicon or silicon compound film byilluminating it with a laser beam, the uniformity of the film surface isdegraded. The embodiment described below can suppress such degradationin uniformity, reduce the processing time of laser beam illuminationfrom the case of the two-step illumination that was described in theabove background part of this specification, and attain effectsequivalent or superior to those of the two-step illumination.

First, a laser annealing apparatus will be described. FIG. 1 is aconceptual diagram of a laser annealing apparatus used in thisembodiment. The main part of the laser annealing apparatus is disposedon a table 1. KrF excimer laser light (wavelength: 248 nm; pulse width:25 ns) is emitted from an oscillator 2. Apparently other excimer lasersand other types of lasers can also be used.

A laser beam emitted from the oscillator 2 is reflected byfull-reflection mirrors 5 and 6, amplified by an amplifier 3, reflectedby full-reflection mirrors 7 and 8, and introduced into an optics 4.

Immediately before entering the optics 4, the laser beam assumes arectangular shape of about 3×2 cm². This laser beam is shaped into along and narrow beam (linear beam) having a length of 10 to 30 cm and awidth of 0.1 to 1 cm by the optics 4. This linear laser beam has a beamprofile that is approximately of a normal-distribution type in the widthdirection as shown in FIG. 3. The laser beam as output from the optics 4has a maximum energy of 1,000 mJ/shot.

The reason for shaping an original laser beam into a long and narrowbeam is to improve the processing ability, as described below. A linearlaser beam output from the optics 4 is applied to a sample 11 via afull-reflection mirror 9. Since the laser beam is longer than the widthof the sample 11, the laser beam can be applied to the entire sample 11by moving the sample 11 in one direction. Therefore, a stage/drivingdevice 10 for the sample 11 can be made simple in configuration and canbe maintained easily. Further, an alignment operation in setting thesample 11 can be facilitated.

The stage 10 to be illuminated with a laser beam is controlled by acomputer, and so designed as to move perpendicularly to a linear laserbeam. Further, if mechanism for rotating, within its plane, a table onwhich the sample 11 is to be mounted is provided, the laser beamscanning direction can be changed conveniently. Since a heater isprovided under the stage 10, the sample 11 can be kept at a prescribedtemperature during laser beam illumination.

FIG. 2 shows an example of an optical path inside the optics 4. A laserbeam input to the optics 4 is passed through a cylindrical concave lensA, a cylindrical convex lens B, and horizontal and vertical flyeyelenses C and D, further passed through cylindrical convex lenses E andF, reflected by a mirror G (which corresponds to the mirror 9 in FIG.1), focused by a cylindrical lens H, and finally applied to the sample11. By moving the lens H vertically with respect to the illuminationsurface, the laser beam profile on the illumination surface can bechanged from a profile close to a rectangular one to anormal-distribution type profile. Since the full-reflection mirror G inFIG. 2 corresponds to the full-reflection mirror 9 in FIG. 1, actuallythe lens H is disposed between the full-reflection mirror 9 and thesample 11.

A description will be made of an example in which a crystalline siliconfilm is formed on a glass substrate by laser illumination according tothe invention. First, a glass substrate (for example, Corning 7059 or1737) of 10 cm×10 cm is prepared. A 2,000-Å-thick silicon oxide film isformed on the glass substrate by plasma CVD using TEOS as a material.This silicon oxide film serves as an undercoat film for preventingimpurities from diffusing from the glass substrate into a semiconductorfilm.

Thereafter, a 500-Å-thick amorphous silicon film is deposited by plasmaCVD or low-pressure thermal CVD. In the case of forming a crystallinesilicon film through annealing by laser beam illumination, it is desiredthat an amorphous silicon film as a starting film have a thickness ofnot more than 1,000 Å. This is because if the amorphous silicon film isthicker than 1,000 Å, desired annealing effects cannot be obtained.

The amorphous silicon film is formed on the glass substrate in the abovemanner. Then, by using the apparatus shown in FIG. 1, a KrF excimerlaser beam (wavelength: 248 nm; pulse width: 25 ns) is applied to theamorphous silicon film, so that it is converted into a crystallinesilicon film.

The laser beam is shaped into a linear form by the beam shape convertinglenses to provide a beam area of 125 mm×1 mm on the illumination object.Since the linear laser beam has a normal-distribution type profile, thebeam edge is indefinite. In this specification, a beam is defined as aportion of a beam profile in which the energy is not less than 5% of themaximum energy.

The sample 11, which is mounted on the stage 10, is illuminated entirelyby moving the stage 10 at 2 mm/s. As for the laser beam illuminationconditions, the laser beam energy density is set at 300 mJ/cm² and thepulse rate (the number of pulse emissions per second) is set at 30pulses/s. It is noted that the term “energy density” as used hereinmeans an energy density value of a top portion of a beam profile that isclose to a normal distribution.

With the above conditions, V=2×10⁻³ m/s, N=30 s⁻¹, and L=1×10⁻³ m.Therefore, LN/V=15, which satisfies the condition disclosed in thisspecification.

Where laser beam illumination is performed under the above conditions,an arbitrarily selected point (linear region) on the sample 11 isilluminated with 15 laser beams. The 15 laser beam pulses have energydensity values corresponding to values E₁ to E₁₅ of anormal-distribution type profile shown in FIG. 4. If linear laser beamsare used under the above conditions, laser beam pulses having energydensity values of E₁ to E₁₅ are sequentially applied to cover a lineararea of 125 mm×1 mm.

FIG. 3 shows how linear laser beams are applied while being moved forscanning. When attention is paid to, for instance, a linear region A, itis seen that first it is illuminated with a laser beam pulse having alow energy density corresponding to a foot portion of a normaldistribution and then the energy density of applied laser beam pulsesgradually increases. When attention is paid to a linear region B, it isseen that after it is illuminated with a laser beam pulse having amaximum energy density corresponding to a top portion of the normaldistribution, the energy density of applied laser beam pulses graduallydecreases. Therefore, the illumination energy can be optimized withoutchanging the output of the laser oscillator 2. Since the laseroscillator 2 can always be rendered stable, uniform laser annealing isassured.

Experiments of the inventors have revealed that best crystalline siliconfilms can be obtained when the parameter LN/V is in a range of 10 to 30.That is, the best condition for crystallizing a silicon film is toilluminate its prescribed linear region 10 to 30 times. The energydensity of illumination laser beams should be in a range of 100 to 500mJ/cm², preferably 300 to 400 mJ/cm².

The substrate temperature is kept at 200° C. during laser beamillumination to reduce the speed of increase and decrease of thesubstrate surface temperature due to the laser beam illumination. It isknown that in general an abrupt change in environmental conditionsimpairs the uniformity of a substance. Deterioration of the uniformityof the substrate surface due to the laser beam illumination is minimizedby keeping the substrate temperature high. Although in this embodimentthe substrate temperature is set at 200° C., in practice it is set at atemperature most suitable for laser annealing in a range of 100 to 600°C. No particular atmosphere control is performed; that is, theillumination is conducted in the air.

Embodiment 2

This embodiment is directed to a case of improving, by laser beamillumination, the crystallinity and uniformity of a silicon film thathas already been crystallized by heating. Studies of the inventors haveshown that a crystalline silicon film can be obtained by a heattreatment of about 550° C. and 4 hours by adding a metal element foraccelerating crystallization of silicon. This technique is described inJapanese Unexamined Patent Publication Nos. Hei. 6-232059 and Hei.244103.

By using this technique, a crystalline silicon film can be formed evenon a large-area glass substrate in a temperature range where strain etc.cause no serious problem. Thin film transistors produced by using such acrystalline silicon film have characteristics far superior to those ofconventional thin-film transistors produced by using an amorphoussilicon film. Specifically, while the mobility of thin-film transistorsusing an amorphous silicon film is less than 1 cm²/Vs, that of thin-filmtransistors produced by utilizing the above crystallizing techniqueinvolving the use of a metal element is more than several tens ofcm²/Vs.

However, observations by taking electron microscope photographs and byRaman spectroscopy have revealed that many amorphous components remainin a crystalline silicon film produced by using the above technique. Ithas also been proved that by crystallizing the remaining amorphouscomponents by laser beam illumination, the characteristics of resultingthin-film transistors can further be improved.

A description will be made of a process for manufacturing a crystallinesilicon film according to this embodiment. First, a 2,000-Å-thicksilicon oxide film as an undercoat film is deposited on a glasssubstrate. A 500-Å-thick amorphous silicon film is deposited thereon byplasma CVD. A nickel acetate salt solution is then applied to thesurface of the amorphous silicon film with spin coater. The nickelconcentration in the nickel acetate salt solution is so adjusted thatnickel will finally remain in a silicon film at a concentration of1×10¹⁶ to 5×10¹⁹ cm⁻³. This is because if the nickel concentrationexceeds the above range, properties as a metal silicide appear, and ifit is lower than the above range, the effect of acceleratingcrystallization is not obtained. The nickel concentration is defined asa maximum value of a SIMS (secondary ion mass spectrometry) measurement.

In terms of reproducibility and effects, nickel is most advantageous asthe metal element for accelerating crystallization of silicon. However,there may also be used one or a plurality of elements selected from Fe,Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. In particular, Fe, Cu and Pd, andeven Pt can provide sufficient effects for practical use.

In the state that nickel is retained adjacent to the surface of theamorphous silicon film, the substrate is left at 450° C. for 1 hour in anitrogen atmosphere, to remove hydrogen from the amorphous silicon film.This heat treatment is conducted to reduce the threshold energy of alater performed crystallization step by intentionally forming danglingbonds.

Thereafter, a heat treatment of 550° C. and 4 hours is performed in thestate that nickel is retained adjacent to the surface of the amorphoussilicon film, to convert the amorphous silicon film into a crystallinesilicon film. Although this heat treatment may be performed at atemperature higher than 500° C., it is important that the temperature belower than the strain point of the glass substrate.

Thus, a crystalline silicon film is formed on the glass substrate. Laserbeams are then applied to the silicon film by the same method and underthe same conditions as in the first embodiment. As a result, acrystalline silicon film further enhanced in crystallinity anduniformity is obtained. It has been revealed by experiments that thelaser beam illumination is more effective than in the first embodimentif the energy density of laser beams is increased by 20 to 50%.

As is the case of this embodiment, the technique of improving, by laserbeam illumination, the crystallinity of a silicon film crystallized by aheat treatment with addition of a metal element for acceleratingcrystallization of silicon can provide a crystalline silicon film thatis superior in crystal quality and uniformity, and productivity to thatproduced only by heating or laser beam illumination.

Where laser light is applied, by an ordinary method, to a crystallinesilicon film that has been produced by heating with introduction of ametal element such as nickel, there occur such phenomena as segregationand partial cohesion of the metal element. Segregation and partialcohesion of the metal element, which cause trap centers, are factors ofgreatly deteriorating the electrical characteristics of resultingsemiconductor devices. In contrast, no such phenomena are found if thelaser beam illumination method of this invention is employed. This isbecause the segregation and partial cohesion of the metal element can besuppressed by gradually increasing the laser beam energy starting from alow level.

Embodiment 3

While in the first and second embodiments linear laser beams having anormal-distribution type energy profile are applied, in this embodimentlinear laser beams having a trapezoidal energy profile are applied. Alsoin this embodiment, a crystalline silicon film is annealed byilluminating it with KrF excimer laser beams (wavelength: 248 nm; pulsewidth: 25 ns) by using the laser illumination apparatus shown in FIGS. 1and 2.

A laser beam is shaped into a linear form by the optics 4, to have abeam area of 125 mm×1 mm on an illumination object. The energy profilein the width direction of the linear form is trapezoidal as shown inFIG. 5. Due to the nature of such a beam profile of the linear laserbeam, the beam edge is indefinite. In this specification, a beam isdefined as a portion of a beam profile in which the energy is not lessthan 5% of the maximum energy.

The sample 11, which is mounted on the stage 10, is illuminated entirelywith linear laser beams by moving the stage 10 at 2 mm/s. As for thelaser beam illumination conditions, the laser beam energy density is setat 100 to 500 mJ/cm² and the pulse rate is set at 30 pulses/s. It isnoted that the term “energy density” as used herein means an energydensity value of a top base portion (having a maximum value) of atrapezoidal beam profile.

If laser beam illumination is performed under the above conditions, inwhich the width of pulse laser beams is 1 mm and it takes 0.5 second forthe sample 11 to pass the 1-mm-wide area, a point on the illuminationsurface receives 15 laser beam pulses. That is, an arbitrarily selectedpoint on the sample 11 is illuminated with 15 laser beams in one scan.In this embodiment, since linear laser beams having a trapezoidal energyprofile are applied, the illumination energy density increases in firstseveral beam applications in one scan and decreases in last several beamapplications.

This is schematically illustrated in FIG. 5. The laser beam energygradually increases in the first half of the 15 beam applications (payattention to portion A in FIG. 5), and gradually decreases in the lasthalf (portion B in FIG. 5). Therefore, the illumination energy can beoptimized without changing the output of the laser oscillator 2. Sincethe laser oscillator 2 can always be rendered stable, uniform laserannealing is assured. The number 15 can easily be calculated from thelaser beam width, the speed of the stage 10, and the number of laserbeam pulses. According to our experiments, a silicon film having bestcrystallinity can be produced by 3 to 100 beam applications, preferably10 to 20 beam applications.

The substrate temperature is kept at 500° C. during laser beamillumination to reduce the speed of increase and decrease of thesubstrate surface temperature due to the laser beam illumination. It isknown that in general an abrupt change in environmental conditionsimpairs the uniformity of a substance. Deterioration of the uniformityof the substrate surface due to the laser beam illumination is minimizedby keeping the substrate temperature high. Although in this embodimentthe substrate temperature is set at 500° C., in practice it is set at atemperature most suitable for laser annealing in a range of 400° C. tothe strain point of the glass substrate. No particular atmospherecontrol is performed; that is, the illumination is conducted in the air.

Fourth Embodiment

A fourth embodiment is characterized in that an energy profile of anirradiated laser beam has a shape shown in FIG. 6 by devising an opticalsystem.

In annealing due to the irradiation of a laser beam onto a silicon film,it is preferable that an irradiated energy density is graduallyincreased in a region to be irradiated, and further the irradiatedenergy density is gradually lowered.

This is because the effect of annealing is unified, and further theroughness of the surface of a silicon film, which is caused byannealing, can be restrained.

The irradiated energy profile shown in FIG. 6 is a profile in a casewhere an amorphous silicon film is crystallized by the irradiation of alaser beam.

The irradiated energy profile shown in FIG. 6 represents a section of alaser beam which is shaped linearly. In FIG. 6, a laser beam isirradiated on an object to be irradiated while scanning the object fromright to left.

It should be noted that, in FIG. 6, the axis of ordinate represents arelative value of a normalized irradiated energy density. The axis ofabscissa represents the widthwise direction of a laser beam which isshaped linearly.

Hereinafter, a laser annealing process will be described with referenceto a process of irradiating a laser beam on an amorphous silicon film tochange the amorphous silicon film into a crystalline silicon film as anexample.

In the case where attention is paid to a certain specified region to beirradiated, a portion 603 of a laser beam is irradiated onto the regionto be irradiated. In this situation, the energy density of a portion 603of a laser beam is set to such a degree that the amorphous silicon filmis not heated. With such a density, preheating is conducted before theamorphous silicon film is crystallized.

Subsequently, as a laser beam scans the object to be irradiated, theportion 602 of a laser beam is irradiated onto the region to beirradiated.

The portion 602 of a laser beam is set so as to have an energy densitythat allows the amorphous silicon film to be melted and crystallized.

With such a setting, the amorphous silicon film can be crystallized. Inthis situation, since heating conducted by the irradiation of theportion 602 of a laser beam is not rapid, the crystallization can beconducted with an excellent uniformity and the restrained generation ofdefects.

Thereafter, the portion 601 of a laser beam is irradiated onto theregion to be irradiated by scanning the object by a laser beam.

The portion 601 of a laser beam has the same energy density as that ofthe portion 603 of a laser beam. In other words, the portion 601 of alaser beam has an irradiated energy density to such a degree that thesilicon film is heated without being crystallized.

With the irradiation of the portion 601 of a laser beam the silicon filmwhich has been crystallized can come to a state where it is not rapidlycooled.

With the irradiation of the portion 601 of a laser beam, a time ofperiod required for solidifying the silicon film which has been meltedonce can be lengthened.

The portion 601 of a laser beam functions to set the energy density inaccordance with the shape of a beam or the conditions of annealing.

With such a function, crystal growth can be unified. Also, theoccurrence of defects or a stress can be restrained. Further, thegeneration of roughness (concave/convex) of the surface can berestrained.

In the above-mentioned processes, heating at a prestage of thecrystallization of the amorphous silicon film in accordance with rapidheating due to the irradiation of a laser beam, heating due to theirradiation of a laser beam for crystallizing the amorphous siliconfilm, and heating that allows rapid cooling of the crystallized siliconfilm to be restrained (or heating for lengthening a melting period) canbe realized by one process of irradiating a laser beam.

In the beam profile shown in FIG. 6, the energy densities of irradiatedenergies of the portions 601 and 603 of a laser beam are identical toeach other (or the same degree). However, with the alternation of designof an optical system, the beam sectional shape can be alteredopportunely.

Furthermore, as shown in FIG. 7, a portion 701 of a laser beam having anirradiated energy density for crystallization may be increased in widthso that a high irradiated energy is irradiated on a predetermined regionto be irradiated for a longer period.

Such an alternation of design can be conducted opportunely in accordancewith a material to be irradiated, its film thickness, the scanning rateof a laser beam, and annealing effect as required.

In this example, a case where the amorphous silicon film is crystallizedwas described as one example. However, as other examples, this structurecan be used for annealing of the silicon film which has been madeamorphous by implantation of impurity ions, the acceleration of thecrystallization of the silicon film which has been crystallized byheating.

The laser illumination technique of the invention can improve theproductivity and the uniformity of a film to be used for semiconductordevices. While the invention can be applied to any laser processing stepused for a semiconductor device manufacturing process, it isparticularly advantageous in providing superior characteristics and gooduniformity when applied to a thin-film transistor manufacturing process.When illuminating a desired region plural times with laser beams ofdifferent illumination energy densities, the invention can preventmisregistration between laser beam application areas. This is veryeffective in obtaining uniform device characteristics.

The methods of the invention can greatly improve the operationefficiency of a step in which the energy density of laser beams appliedto a desired region is gradually changed in plural beam applications.That is, the invention can provide, by the illumination in one laserbeam scan, the effects similar to those as obtained by the conventionallaser beam illumination method in which laser beams are applied inplural scans while their energy density is changed in a step-likemanner. Therefore, the multi-step illumination can be performed withoptimum illumination energy without changing the output of the laseroscillator. Since the laser oscillator can always be rendered stable,uniform laser annealing is assured.

1. A method of manufacturing a thin film transistor comprising: forming a semiconductor film including amorphous silicon having a thickness of 150 to 1000 Å over a substrate; emitting a pulse laser light at a rate of N pulses per second; shaping the pulse laser light into a beam having a cross section perpendicular to a propagation direction of the beam, said cross section having a width and a length, wherein the length is longer than the width, wherein the beam has a normal-distribution type energy profile of width L (m) perpendicular to the length direction, where L is larger than zero, and the beam having substantially a constant energy distribution along a lengthwise direction; applying the beam to a portion of the semiconductor film; and scanning the semiconductor film with the beam perpendicular to the lengthwise direction of the cross section at a speed V (m/s) in order to crystallize said semiconductor film, wherein the number of beams applied to said portion in one scan satisfies a relationship 3≦LN/V≦100, and wherein the width L (m) is defined as a beam in a region having 5% or more of an energy density with respect to a maximum energy density of the beam on the irradiation surface.
 2. The method of claim 1, wherein the width is 0.1 to 1 cm.
 3. The method of claim 1, wherein the length is 10 to 30 cm.
 4. The method of claim 1, wherein the scanning step is conducted in air.
 5. The method of claim 1, an average single-pulse energy density of the beam is set at 100 to 500 mJ/cm².
 6. The method of claim 1, wherein the pulse laser comprises an excimer laser.
 7. The method of claim 1, wherein the number of beams applied to said portion in one scan satisfies a relationship 10≦LN/V≦30.
 8. A method of manufacturing a thin film transistor comprising: forming a semiconductor film including amorphous silicon having a thickness of 150 to 1000 Å over a substrate; crystallizing the semiconductor film by performing a heat treatment to the semiconductor film; emitting a pulse laser light at a rate of N pulses per second; shaping the pulse laser light into a beam having a cross section perpendicular to a propagation direction of the beam, said cross section having a width and a length, wherein the length is longer than the width, wherein the beam has a normal-distribution type energy profile of width L (m) perpendicular to the length direction, where L is larger than zero, and the beam having substantially a constant energy distribution along a lengthwise direction; applying the beam to a portion of the crystallized semiconductor film; and enhancing a crystallinity of the crystallized semiconductor film by scanning with the beam perpendicular to the length direction of the cross section at a speed V (m/s), wherein the number of beams applied to said portion in one scan satisfies a relationship 3≦LN/V≦100, and wherein the width L (m) is defined as a beam in a region having 5% or more of an energy density with respect to a maximum energy density of the beam on the irradiation surface.
 9. The method of claim 8, wherein the width is 0.1 to 1 cm.
 10. The method of claim 8, wherein the length is 10 to 30 cm.
 11. The method of claim 8, wherein the scanning step is conducted in air.
 12. The method of claim 8, an average single-pulse energy density of the beam is set at 100 to 500 mJ/cm².
 13. The method of claim 8, wherein the pulse laser comprises an excimer laser.
 14. The method of claim 8, wherein the number of beams applied to said portion in one scan satisfies a relationship 10≦LN/V≦30.
 15. A method of manufacturing a thin film transistor comprising: forming a semiconductor film including amorphous silicon having a thickness of 150 to 1000 Å over a substrate; emitting a pulse laser light at a rate of N pulses per second; shaping the pulse laser light into a beam having a cross section perpendicular to a propagation direction of the beam, said cross section having a width and a length, wherein the length is longer than the width, wherein the beam has a trapezoidal energy profile of width L (m) perpendicular to the length direction, where L is larger than zero, and the beam having substantially a constant energy distribution along a lengthwise direction; applying the beam to a portion of the semiconductor film; and scanning the semiconductor film with the beam perpendicular to the lengthwise direction of the cross section at a speed V (m/s) in order to crystallize said semiconductor film, wherein the number of beams applied to said portion in one scan satisfies a relationship 3≦LN/V≦100, and wherein the width L (m) is defined as a beam in a region having 5% or more of an energy density with respect to a maximum energy density of the beam on the irradiation surface.
 16. The method of claim 15, wherein the width is 0.1 to 1 cm.
 17. The method of claim 15, wherein the length is 10 to 30 cm.
 18. The method of claim 15, wherein the scanning step is conducted in air.
 19. The method of claim 15, an average single-pulse energy density of the beam is set at 100 to 500 mJ/cm².
 20. The method of claim 15, wherein the pulse laser comprises an excimer laser.
 21. The method of claim 15, wherein the number of beams applied to said portion in one scan satisfies a relationship 10≦LN/V≦30.
 22. The method of claim 15, wherein the beam has substantially a constant energy distribution along the direction.
 23. A method of manufacturing a thin film transistor comprising: forming a semiconductor film including amorphous silicon having a thickness of 150 to 1000 Å over a substrate; crystallizing the semiconductor film by performing a heat treatment to the semiconductor film; emitting a pulse laser light at a rate of N pulses per second; shaping the pulse laser light into a beam having a cross section perpendicular to a propagation direction of the beam, said cross section having a width and a length, wherein the length is longer than the width, wherein the beam has a trapezoidal energy profile of width L (m) perpendicular to the length direction, where L is larger than zero, and the beam having substantially a constant energy distribution along a lengthwise direction; applying the beam to a portion of the crystallized semiconductor film; and enhancing a crystallinity of the crystallized semiconductor film by scanning with the beam perpendicular to the lengthwise direction of the cross section at a speed V (m/s), wherein the number of beams applied to said portion in one scan satisfies a relationship 3≦LN/V≦100, and wherein the width L (m) is defined as a beam in a region having 5% or more of an energy density with respect to a maximum energy density of the beam on the irradiation surface.
 24. The method of claim 23, wherein the width is 0.1 to 1 cm.
 25. The method of claim 23, wherein the length is 10 to 30 cm.
 26. The method of claim 23, wherein the scanning step is conducted in air.
 27. The method of claim 23, an average single-pulse energy density of the beam is set at 100 to 500 mJ/cm².
 28. The method of claim 23, wherein the pulse laser comprises an excimer laser.
 29. The method of claim 23, wherein the number of beams applied to said portion in one scan satisfies a relationship 10≦LN/V≦30.
 30. The method of claim 23, wherein the beam has substantially a constant energy distribution along the direction. 